PR CI: also run clippy tests

[RalfJung]

The PR CI runner currently does not seem to run clippy tests. Would be nice if it would. :)

Cc @Mark-Simulacrum @rust-lang/clippy

[RalfJung]

This had some unintended side-effects: in a rollup like #79056 that touches a clippy file, PR CI runs the "tools" builder, including Miri -- and then PR CI could fail because Miri is broken, even though the PR would land just fine as it is okay for Miri to be broken. (The PR did not update the Miri submodule.)